Wireless local area networks (WLANs) have invoked great interests from both consumers and the industry. The current most popular WLANs are based on the [Non Patent Document 1] standards. While these standards have helped the initial uptake of WLANs, in their current form, they are not suited for large-scale wireless network deployments. This is because the cost and control of WLAN entities become complex in large environments.
Currently, many WLAN equipment manufacturers have addressed large-scale deployments by introducing new split architecture. Here, control aspects of the [Non Patent Document 1] WLAN specifications are centralized at controller nodes (CNs) while other aspects are distributed to numerous wireless access points (WAPs). With the diversity of manufacturers and their implementations of the split architecture, there are incompatibilities between WLAN entities from different manufacturers.
There are currently some efforts to provide standardized means for managing large-scale WLANs in the Internet Engineering Task Forces (IETF) Control and Provisioning of Wireless Access Point (CAPWAP) working group. [Non Patent Document 2] describes the efforts of the CAPWAP working group. However these efforts do not consider the problems of accommodating WAPs with dissimilar functional capabilities within a single WLAN. As such these problems limit the development of the WLAN market.
Furthermore, it is expected that future deployments of WLANs will feature dynamic wireless networks. In such types of deployments, network topologies will change during the operational lifecycle of the WLAN to enable enhanced applications and services. WLAN elements in such networks will be provisioned with both wired and wireless connectivity to enable dynamic topologies. However current assumptions of WLANs (and also CAPWAP) only refer to static network topologies. So while current WLANs are capable of adjusting to the dynamic conditions of the wireless medium, they are unable to accommodate the effects of dynamic topology changes.
For example, current WLAN systems adjust to declines in the signal-to-interference ratio (SIR) of the wireless medium by increasing the signal transmission power. However such minor corrections are inadequate to accommodate the variances in latency and overhead introduced by changes in WLAN topology. Furthermore, these variances in latency and overhead impede the operation of the CAPWAP split architecture. This is because the split architecture is sensitive to delays due to the very nature of the distributed operations. The redundancies of WLAN and CAPWAP processing performed at intermediate wireless access points (WAP) of a dynamic CAPWAP topology together with the corresponding physical overheads are detrimental to the CAPWAP split operations.
Given such scenarios, WLAN entities currently available from various vendors are incapable of interoperation in a single WLAN and are also incapable of operation in a dynamic topology WLAN.
These problems refer to static differences between WLAN entities as they are results of differences in basic design. In addition to these, there are also problems related to dynamic differences between WLAN entities.
In particular, during the functioning of a WLAN, the processing load at a WAP can become substantially high even exceeding the processing capacity of the WAP. This could be due to increases in the number of associated mobile terminals (MTs) or due to increases in the volume of traffic from the associated MTs. These differences in processing load over time constitute a dynamic factor as they are dependent on the dynamics of the MTs.
These dynamic differences in processing load across the WAPs consisting of a WLAN have traditionally been addressed by affecting handovers of MTs from their associated WAPs where processing load is high, to re-associate the MTs with other WAPs where processing load is relatively low.
[Patent Document 1] discloses means for addressing dynamic differences in the levels of processing load at WAPs by means of proactive handovers of associated MTs. While [Patent Document 1] addresses the problem of dynamic differences in processing loads across WAPs, it does so by mandating that MTs associated with one WAP also be within the coverage areas of other WAPs so as to be able to perform handovers and re-associations. If a MT is not within the coverage area of one or more other assisting WAPs, it is then expected to physically displace to such a coverage area in order to relieve the first WAP of some processing load. These constraints are rigid and limit the efficacy of [Patent Document 1]. Such limitations are common to all handover-based methods.
[Patent Document 2] presents a method for WAPs to modify, based on prevailing processing load levels, the intervals between the beacon signals that they transmit in order to attract or dissuade MT associations. This method also involves the constraints of requiring a MT to be within the coverage areas of alternate WAPs where processing load is low or being agreeable to displace towards such areas.
[Patent Document 3] focuses on proactive MTs that make association decisions. However the method is also limited by the factors described earlier.
While such methods attempt to solve the problem of dynamic differences in processing load, they do so by introducing stringent prerequisites and thereby introduce more problems. Another shortcoming of [Patent Document 1], [Patent Document 2], [Patent Document 3] and other handover-based methods for dealing with dynamic differences in WAPs is related to the bulk shifting of communication sessions. In practice MTs maintain a number of communication sessions with the WAPs with which they are associated. As a result, it is very likely that the communication sessions of only one MT or a few MTs constitute a considerable amount of processing load at the WAP. If the WAP were to affect the said MTs to handover and re-associate with another WAP, the processing load at the first WAP would be reduced, however by adversely affecting the other WAP. The other WAP then becomes overloaded and reverses the handover to the first WAP. This may continue without delivering any net gains for the WLAN. This points out that the processing load is not finely distributed by methods of handovers. In other words, dynamic differences are not finely managed.
[Non Patent Document 1] Institute of Electrical and Electronics Engineers Standard 802.11-1999 (R2003)
[Non Patent Document 2] “CAPWAP Problem Statement”, draft-ietf-capwap-problem-statement-02.txt
[Patent Document 1] “Method and apparatus for facilitating handoff in a wireless local area network”, US 2003/0035464 A1
[Patent Document 2] “Dynamically configurable beacon intervals for wireless LAN access points”, US 2003/0163579 A1
[Patent Document 3] “Method and apparatus for selecting an access point in a wireless network”, U.S. Pat. No. 6,522,881 B1